Caption: Fishes killed due to a lack of water oxygen from river pollution. Levels of pollution in a stream or river can be monitored both by analysing the flora and fauna present, and the concentration of dissolved oxygen. Including discharge of chem- ical effluents from industry into rivers, and the use of farming fertilizers which run off into streams. Such fertilizer run off from agricultural land causes phosphate and nitrogen enrichment of rivers and lakes (or eutrophication). Once-clear waters become turbid with microscopic algae (as seen here) which live off the enriched conditions, causing an oxygen depletion and death for the other flora and fauna of the water system.